1. Field of the Invention
The present invention relates to a process and apparatus for production of diamond, more particularly relates to a process and apparatus for production of diamond which performs gas phase synthesis of high quality diamond at a high speed.
2. Description of the Related Art
Diamond is the hardest material on earth, having a Vicker's hardness of 10000. It also has a large Young's modulus and is superior in wear resistance and chemical stability. Due to these superior properties, diamond is an essential material in industry, in particular the high tech industries. Further, diamond has a heat conductivity of 2000 W/mK, four times that of copper, and is transparent in a wide range of wavelength from the infrared to ultraviolet. Further, it is a semiconductor with a wide bandgap of 5.4 eV. Therefore, diamond is expected to be put to use for heat sinks, heat radiating circuit boards, optical materials (window materials), heat resistant semiconductor devices, wear resistant coatings for tools and other objects, etc.
In recent years, there has been considerable research done in the synthesis of diamond by the chemical vapor deposition method (CVD method). For commercial use of gas phase synthesized diamond, it is important to increase the speed of synthesis so as to reduce the manufacturing cost.
Many processes have been proposed as the process for gas phase synthesis of diamond. By the DC plasma jet CVD method developed by the present inventors (see Japanese Unexamined Patent Publication (Kokai) No. 64-33096), it is possible to synthesis diamond at a high production or film-forming speed of over 100 .mu.m/hour.
FIG. 1 is a view for explaining the structure of a plasma torch of a conventional diamond producing apparatus.
In FIG. 1, 1 is circular columnar cathode with a pointed front end, and 2 is an anode provided so as to surround the cathode 1 with a predetermined clearance. By applying DC voltage between the cathode 1 and the anode 2, an arc discharge is caused and reaction gas introduced in the clearance between the cathode 1 and anode 2 is converted to plasma in the clearance. Reference numeral 3 is a gas introduction port for introduction of the reaction gas in the clearance between the cathode 1 and the anode 2.
When using a production apparatus having the above plasma torch to synthesize diamond, a substrate 7 is placed at a position facing the plasma torch, reaction gas is introduced from the gas introduction port 3 in the clearance between the cathode 1 and the anode 2, voltage is applied between the cathode 1 and the anode 2 to cause an arc discharge, and the reaction gas introduced in the clearance is converted to plasma. Due to this, an arc column 5 is produced from the front end of the cathode 1 to the jetting port of the plasma jet 6 of the anode 2.
The plasma jet 6 is jetted from the jetting port of the front end of the plasma torch toward the substrate 7. This state is held for a predetermined time, whereby a diamond film 8 is formed on the substrate 7.
In general, with a DC arc discharge, even with the same discharge power, if the arc column 5 is made longer, the discharge voltage raised, and the discharge current lowered, there is less consumption of the electrode material of the anode 2 and the rate of voltage fluctuation also drops. Further, in synthesis of diamond, in the case of the same discharge power, the higher the discharge voltage is made, the faster the speed of synthesis becomes as a general trend (see FIG. 2). In this way, lengthening the arc column and raising the discharge voltage are very effective in reducing the electrode consumption of the cathode 1 and the anode 2, improving the stability of the discharge, raising the purity of the synthesized diamond film, improving the quality, and raising the speed of synthesis.
However, if the arc column 5 is made too long, the arc easily disappears and so there were limits to the conventional method of using a DC arc discharge. That is, there were the problems that the electrode materials of the cathode 1 and the anode 2 easily were mixed in the diamond film 8 as impurities or that the discharge was unstable and there was insufficient reproducibility. Further, the angle of spread of the plasma jet was narrow and only a small area of synthesis was possible. In this way, it was difficult with the conventional process to stably synthesize a high quality diamond film 8 over a large area.